If yer talking longbows specifically, the Hill or mild r/d styles require longer lengths because there's little or no riser deflex. Aggressive r/d longbows are closer to recurves and one can go shorter.
At my 29" draw, a 60" aggressive hybrid is as low as I can go without sacrificing "stability & shootability", and it takes a 62" mild r/d to equate the same drawing/holding/shooting characteristics.
The really short hyper r/d longbows (Shrews, for example) are in a class by themselves and can be the missing link answer if yer want a really short, but effective longbow at a longer than 28" draw length.